A flow cell is an apparatus for characterization of particles suspended in a sample solution. Particles sizes are generally in the range of ˜0.5-40 μm. Particles are analyzed one-by-one with a typical count rate in the range of a few to thousand particles per second. Depending on its configuration, a flow cell could allow estimating different information about the particles such as presence, concentration, dimension, shape, vitality (in the case of cells), types of cells, structural and/or functional information, etc. Using a flow cell for sorting particles of different types in a heterogeneous solution is also possible.
Flow cytometers, which incorporate different configurations of flow cells, have been developed over the last 40 years. In general, a light source (i.e. a laser) emitting a light beam is focused on a fluid stream in the flow cell. The fluid flows at a predetermined rate in a capillary tube of the flow cell. Particles in the fluid stream cross the light during a brief interval of time, hence forming a short burst of temporal scattered light and fluorescence. A collection optics assembly, localized near or around the region where light and fluid intersect collects light emitted and/or scattered by the particles. The collected light is spectrally separated by a detection subassembly system, including for example various optical filters, and then received by detectors. Optical signal parameters of the collected light are measured by the detectors, and are processed by a computational system and/or electronic components.
For more than four decades, Gaussian optical pulses have been used in flow cytometry. The Gaussian optical pulses are the result of flow cell and flow cytometer design constraints: the use of spatially narrow laser beams dictated by optical density required for sufficient detection sensitivity; the spatial beam distribution of the laser, which is inherently Gaussian in shape, is translated in Gaussian pulses when particles transit at constant velocity through the beam; the requirement for high pulse rate generation hence short pulses to increase the throughput of the flow cytometer; noise source dominated by electrical noise; and analog circuitry that was well suited to perform Gaussian pulse filtering, followed by analog pulse detector, analog baseline tracking, peak detectors, log amplifiers and analog samplers, etc.
However, Gaussian optical pulses require important analog and digital treatment and signal processing resources, to extract characteristics of the particles in the solution. These required resources result in more complex and expensive flow cytometers. Furthermore, current flow cells and flow cytometers are limited by the inherent design constraints related to Gaussian signal generation, namely precise alignment of the laser beam with the position of the transiting particles, which is prone to misalignment and results in frequent and tedious alignment procedures for the users. There is therefore a need for an improved flow cell and method for characterizing particles in a solution by means of non-Gaussian temporal pulses to mitigate or eliminate these drawbacks.